In a circuit which converts an alternating voltage of alternating voltage power supply to a direct voltage on the basis of the alternating voltage, an alternating voltage flowing to the alternating voltage power supply becomes a sine-wave shape in phase with voltage of the alternating voltage power supply. In such a circuit, a phase factor is the most efficient, and no harmonic noise occurs. Such a circuit that performs a control so that an input current becomes a sine wave is called as a PFC (Phase Factor Collection). Hereinafter, the function is referred to as the PFC.
The PFC controls a circuit current so that a sine wave current in phase with an alternating voltage phase of the alternating power supply flows. Thus, a module of detecting the circuit current is necessary.
However, a position where the circuit current can be detected is a position potential at which is different from GND potential of the circuit, and thus a module of insulating a signal is necessary. In general, an insulation current transformer is used to realize insulation of the signal.
Further, even if the current can be detected by the current transformer, the signal is weak and a malfunction caused by noise mixing always occurs. Thus, an implementation for acquiring a signal at a position nearest to a control GND is desired, if possible.
Further, in order to perform a control with certainly, if the current transformer tries to detect not only a 50 Hz component of the alternating voltage power supply but also a 50 kHz component of the switching frequency, a special part of which range of frequency characteristics is wide is necessary for a core used in the current transformer. If the part is mounted, the size of the current transformer becomes large and the price thereof is undesirably higher than a general core.